In regard to NA6, the NB's have improved suspension geometry, larger/more stout motor, stronger differential, larger brakes, better aero, etc. etc.
The above improvements, along with it being at a minimum 6-7 years newer than the NA6 chassis will influence the overall price. (It's newer, it has to be better/faster right?)
And of course, as others have said, if it has a good racing pedigree then you can typically ask for more since the chassis/motor is proven.
In addition, even a perfect street car with low mileage can go for under $10K (at least in CA) and many other clean examples in the 3-7k range.
That is still a reasonable price which appeals to a lot of people, so the demand for these cars has and will continue to rise.
Do I agree with the inflation of NB prices? If I'm selling/plan to sell then absolutely. Complete opposite if you're planning to buy one that's already built.
However, racing has and always will be an expensive sport/hobby. As they say, "you gotta pay to play" .